SANTA ANA, Calif. - An explosion Tuesday in the boiler room of a high school gymnasium was so powerful it lifted the building's roof and caused heavy damage inside but no serious injuries.

The blast, around 7:30 a.m. at Valley High School in Santa Ana, was in a girls locker room and happened before classes began, although some students already were on campus.

Security camera video shows the fiery explosion lift the building's roof as at least a half-dozen people ran from the area. The blast blew doors off their hinges and sent smoke billowing from the building, which houses the gym, locker rooms and offices.

No students or staff members were inside, said Deidra Powell, a spokeswoman for the Santa Ana Unified School District.

Four students sitting on a bench with their backs to the boiler room were taken to a hospital for injuries described as minor.

"They were more just shook up," Orange County Fire Authority spokesman Capt. Jon Muir said.

Muir said the explosion ripped off a door and blew it about 75 feet, tore at least three other doors from their hinges, knocked down ceiling tiles, and lifted the roof 4 to 5 inches off its load-bearing wall. The cause is under investigation.